Static eliminator, and microphone electretizing method and apparatus using static eliminator

ABSTRACT

The static eliminator includes an ionizer including a power source and an electrode which applies ions to an electretized dielectric film to eliminate electric charges formed on the electretized dielectric film. The electrified substance is in an electrically non-grounded state when the electric charges on the electretized dielectric film are eliminated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/JP2009/002493 filed on Jun. 3, 2009, which claims priority toJapanese Patent Application No. 2008-147023 filed on Jun. 4, 2008, theentire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an static eliminator and a microphoneelectretizing method and apparatus, and more particularly to reductionsof a sensitivity change accompanied by a change in an amount of electriccharges retained in a dielectric film charged by electretization, thechange in the amount being caused by applying ions from an ionizer tothe dielectric film.

BACKGROUND ART

An MEMS microphone is an acousto-electric transducer including anelectret film having semi-permanent polarization which is manufacturedby embedding electric charges through an electretization process,thereby eliminating a need for a direct current bias of a condenser. Theelectret film is produced by injecting electric charges into adielectric film and fixing the thus-injected electric charges. Anelectric field formed by the injected charges causes a potentialdifference across electrodes of the condenser. As used herein, the term“electretization” means a process for injecting electric charges into adielectric film so as to fix the electric charges in the dielectric film(i.e., so as to allow the dielectric film to retain the electriccharges), and a term “retained charge amount” means an amount of fixedelectric charges.

An MEMS (Micro Electromechanical System) microphone manufactured byprocessing a silicon substrate using a semiconductor processingtechnique has recently gained attention. The electretization method usedfor the MEMS microphone is a method for electretizing a dielectric filmin an MEMS microphone chip, which is produced by micromachining asilicon substrate, mounted on a mounting board, or in the MEMSmicrophone chip per se. In the electretization method, one or more MEMSmicrophone chips are subjected to at least one corona discharge processusing one needle electrode or wire electrode, thereby electretizing thedielectric film (Patent Document 1).

As a high-precision electretizing method, there is a method in whichcorona discharge is performed above a fixed electrode while a dielectricfilm of a condenser microphone is set to a ground potential and thefixed electrode is set to an electric potential different from theground potential, thereby electretizing the dielectric film (PatentDocument 2).

Patent Document 1: International Publication WO 2006/132193

Patent Document 2: JP-A-2007-294858

SUMMARY OF THE INVENTION

In an electronic device, influence of electrostatic discharge (ESD)(dielectric breakdown of a circuit system, absorption of an extraneousmatter, etc.) cannot be ignored. For this reason, elimination ofelectric charges on an electrified substance by an ionizer isindispensable.

Accordingly, a static elimination process for eliminating unnecessarycharges is performed, in which ions are applied from the ionizer to anelectretized dielectric film.

However, the applicant has found that the retained charge amount changesby an application of ions from the ionizer to the electretizeddielectric film, according to an experiment. A conceivable reason forthis phenomenon is that electric charges escape from the electretizeddielectric film, in association with the application of ions by theionizer.

Moreover, the retained charge amount is proportional to sensitivity of amicrophone. For this reason, a change in the retained charge amountcaused by applying ions from the ionizer results in a variation insensitivity of the microphone. Therefore, there is a need to reduce thechange in the retained charge amount.

The present invention was made in consideration of the abovecircumstances, and an object thereof is to provide a microphone withreduced sensitivity variations, which can eliminate electric charges onan electrified substance by use of an ionizer while reducing the changein the amount of electric charges retained in an electretized dielectricfilm.

Another object of the present invention is to eliminate unnecessaryelectric charges.

In an aspect of the invention, a static eliminator includes an ionizercomprising a power source and an electrode which applies ions to anelectretized dielectric film to eliminate electric charges formed on theelectretized dielectric film, wherein the electretized dielectric filmis in an electrically non-grounded state when electric charges on theelectretized dielectric film are eliminated.

According to the aspect of the invention, it is possible to reduce achange in a retained charge amount of the electretized dielectric filmwhile a static elimination effect provided by the ionizer is maintained,so that the charges retained in the electretized dielectric film can beless likely to escape, influence of the change in retained electriccharges caused by the ionizer can be reduced, and a variation inmicrophone sensitivity can also be reduced.

The applicant has found that the change in the retained charge amountoccurs only when ions from the ionizer are directly applied to theelectretized dielectric film and also when a grounded conductivesubstance is placed below the dielectric film according to anexperiment. Further, it has been also found that the change in theretained charge amount also occurs even when an insulating substance isinterposed between the dielectric film and the conductive substance.

Accordingly, the conductive substance located below the electretizeddielectric film may be maintained in the non-grounded state such thatthe conductive substance is not connected to a ground. Consequently, itis possible to prevent pull-in of ions to the dielectric film by anelectric field generated when an electric current flows from theconductive substance to the ionizer by the ground connection therebyforming a loop circuit. As a result, the charges retained in thedielectric film are less likely to escape, which can reduce the changein the retained charge amount and a change in microphone sensitivity.

Even when there is provided a structure in which the conductivesubstance is not placed below the electretized dielectric film, anelectric field by formation of the loop circuit is not generated, and anadvantage similar to that described above can be provided.

As used herein, the term “non-grounded state” means anon-potentially-connected state, i.e., a floating state, for preventingan electric current flowing in the dielectric film.

As described above, one aspect of the present invention can prevent theloop circuit formed by flow of the electric current from a conductivesubstance to an ionizer, by not grounding the conductive substancelocated in the vicinity of an electretized dielectric substance.Therefore, it is possible to inhibit occurrence of pull-in of ions tothe dielectric film, which is caused by an electric field generated whenthe loop circuit is formed by flow of the electric current from theconductive substance to the ionizer. Consequently, the change in theretained charge amount caused by the ionizer is reduced, whereby thevariation in the retained charge amount is reduced, which can lead to areduction of the change in sensitivity of a product, e.g., a microphone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of ionizer-installed facilities of anembodiment of the present invention;

FIG. 2 is a schematic diagram showing that processing pertaining to anelectretization process is performed by use of the ionizer-installedfacilities;

FIG. 3 is a view showing an MEMS microphone chip produced in theembodiment of the present invention; and

FIG. 4 is a view showing a mounted state of a microphone.

DETAILED DESCRIPTION OF THE EXEMPLIFIED EMBODIMENTS OF THE INVENTION

An embodiment of the present invention is hereunder described in detailby reference to the drawings. FIG. 1 is a diagrammatic illustration ofionizer-installed facilities of an embodiment of the present invention.FIG. 2 is an illustration of the ionizer-installed facilities in anelectretization process. FIG. 3 is a view showing an MEMS microphonechip. FIG. 4 is a view showing a mounted state of the microphone chip.

FIG. 1 shows a static elimination process of the embodiment intended forstabilizing a retained charge amount of an electretized dielectric film32 and for eliminating unnecessary electric charges. A conductivesubstance 4 such as a chassis of the facilities contacts a mountingboard 42 (an insulating substance) serving as a base for securing anMEMS microphone chip 43 including the electretized dielectric film 32.In the static elimination process, the conductive substance 4 is notconnected to a ground 5 and is held in a non-grounded state. By settingthe conductive substance 4 in the non-grounded state, an electriccurrent does not flow from the conductive substance 4 to an ionizer 1,so that a loop circuit is not formed. For this reason, an electric fielddoes not develop, and pull-in of ions into the dielectric film 32, whichis caused by the electric field, can be prevented. Accordingly, a changein a retained charge amount is prevented, so that a change insensitivity of a microphone can be reduced. The ionizer 1 includes aneedle electrode 11 and a high voltage power source 12 for feedingelectric power to the needle electrode 11. The ionizer 1 functions as astatic eliminator, and whether to function as the static eliminatordepends on whether the conductive substance 4 such as the chassis isconnected to a circuit.

FIG. 2 shows an electretization process for electretizing the dielectricfilm 32 according to the present embodiment. In the electretizationprocess, the conductive substance 4 on which the MEMS microphone chip 43is placed is grounded, and a distance between the ionizer 1 and thedielectric film 32 is set to 14 mm. Further, in a state where the fixedelectrode 31 of the MEMS microphone chip 43 is held in an electricpotential different from the ground potential, and corona discharge isperformed at a point above the fixed electrode 31 by use of the ionizer1, thereby electretizing the dielectric film 32. Thereafter, the staticelimination process is performed as shown in FIG. 1. In the staticelimination process the conductive substance 4 is disconnected from theground 5, and ions are applied by the ionizer 1, thereby eliminatingunnecessary electric charges.

In the electretization process, ions generated by corona discharge reachthe dielectric film 32 via acoustic holes 35 of the fixed electrode 31while controlled by an electric potential of the fixed electrode 31,whereby the dielectric film 32 can be electretized. The dielectric film32 is electretized until the electric potential of the dielectric film32 in the electretization process becomes equal to the electricpotential of the fixed electrode 31. Therefore, the dielectric film 32can be electretized to a target retained charge amount with superioraccuracy and without previously studying a relationship betweenconditions for by electretization and a voltage used for electretizingthe dielectric film 32 (i.e., an electric potential of the dielectricfilm by electretization).

In the above descriptions, the structure in which the conductivesubstance 4 is placed below the material (insulating substance) 3 isexplained as shown in FIG. 1. However, even when a bottom of the chassisis made of a heat resistant insulating substance such as polycarbonateand when the conductive substance 4 located in the vicinity of thedielectric film 32 is removed, a similar advantages can be obtainedbecause an electric field caused by the loop circuit is not formed.

An electretizing method of the embodiment of the present invention isnow described by reference to the drawings.

FIG. 3 is a cross-sectional view of a device for describing a structureof an MEMS microphone chip (a silicon microphone) produced bymicromachining a silicon substrate.

As shown in FIG. 3, the silicon microphone 43 includes: a siliconsubstrate (a silicon diaphragm) 34; a vibrating film 33 that covers aremoval area 38 of the silicon substrate 34 and serves as one ofelectrodes of a condenser; the dielectric film 32 that is provided onthe vibrating film 33 and serves as a film to be electretized; and thefixed electrode 31 that is supported on the silicon substrate 34 by aspacer 37 so as to be opposed to the vibrating film 33 and serves as theother one of the electrodes of the condenser. The fixed electrode 31 hasthe plurality of acoustic holes 35 (openings for guiding acoustic wavesto the vibrating film 33). An air gap 36 formed by etching a sacrificelayer is interposed between the vibrating film 33 and the fixedelectrode 31. The vibrating film 33, the fixed electrode 31, and theinorganic dielectric film 32, which form the silicon microphone 43, areproduced by using a silicon micromachining technique and a CMOS(Complementary Field Effect Transistor) manufacturing process technique.

In the embodiment, the electretization is applied to the MEMS microphonechip.

The ionizer 1 used in the electretizing process shown in FIG. 2 (andalso used in the static elimination process shown in FIG. 1)electretizes the silicon microphone such that the ions are applied bycorona discharge using one needle electrode to one silicon microphone.

As shown in FIG. 2, the corona discharge by the ionizer 1 using theneedle electrode 11 is used for an electretization process of thepresent embodiment. Specifically, the needle electrode 11 is positionedabove the silicon microphone (semiconductor device) 43. The needleelectrode 11 is connected to the high voltage power source 12 forgenerating corona discharge. The high voltage power source 12 applies ahigh voltage, for example, of about 5 to 10 kV, to the needle electrode11. The static eliminator of the embodiment includes the ionizer 1including the needle electrode 11 and the high voltage power source 12for feeding power to the needle electrode, and the requirement of thestatic eliminator includes whether the conductive substance 4 such asthe chassis is connected to a circuit. The electrode is not limited tothe needle electrode and may have any shape as long as the electrode cangenerate corona discharge.

In an electretization process, a wiring connection different from thatdescribed in the embodiment is applied to the silicon microphone 43.

In the above-mentioned state shown in FIG. 2, ions generated by coronadischarge using the needle electrode 11 are applied to the dielectricfilm 32 (see FIG. 3) provided in the silicon microphone 43. Accordingly,the dielectric film 32 (see FIG. 2) of the silicon microphone 43 iselectretized at the voltage applied to the fixed electrode 31.

The fixed electrode 31 is grounded or set to a predetermined electricpotential at this time. In other words, the fixed electrode 31 of thesilicon microphone 43 is set to an electric potential different from theground potential. Accordingly, a potential difference occurs between thevibrating film 33 and the fixed electrode 31. Consequently, the ionsgenerated by the corona discharge using the needle electrode 11 areapplied to the dielectric film 32 via the plurality of acoustic holes 35formed in the fixed electrode 31.

The dielectric film 32 is gradually electretized, so that an electricpotential (an electret potential) of the dielectric film 32 is graduallyincreased. Finally, the electric potential of the surface of thedielectric film 32 on the vibrating film 33 becomes equal to theelectric potential of the fixed electrode 31.

When the electric potential of the dielectric film 32 becomes equal tothe electric potential of the fixed electrode 31, ions are not appliedto the dielectric film 32. Therefore, the dielectric film 32 iselectretized until the electret potential becomes equal to the electricpotential applied to the fixed electrode 31.

The electretization process is performed in a state in which theconductive substance 4 is grounded (see FIG. 2), under conditions wherea distance between the ionizer and the dielectric film is 14 mm, anionizer application time is 2.5 sec., an air flow rate is 0 L/min.

Subsequently, the conductive substance 4 is disconnected from the ground5 as shown in FIG. 1, and then unnecessary electric charges areeliminated (the static elimination process).

The static elimination process is performed in a state in which theconductive substance 4 is grounded (see FIG. 1), under ionizerconditions where a distance between the ionizer and the dielectric filmis 80 mm, an ionizer application time is 180 min., and an air flow rateis 10 L/min. When the conductive substance 4 is connected to the ground5 as in the related art, an average change in the retained charge amountis −0.83 dB (on a sensitivity basis). On the contrary, when theconductive substance 4 is not grounded, the average change in theretained charge amount is confirmed to decrease to −0.04 dB.

FIG. 4 is a cross-sectional view showing a mounting structure of theelectret microphone using the silicon substrate (a structure obtainedafter the microphone is sealed in a case). In FIG. 4, the elementscommon to FIG. 3 are assigned the same reference numerals, and theirrepeated explanations are omitted. Further, FIG. 4 shows a simplifiedsilicon microphone (semiconductor device) 43 but an actual structure ofthe microphone is as illustrated in FIG. 3.

As shown in FIG. 4, the silicon microphone (a semiconductor device) 43and an electronic component (an FET, a resistor, an amplifier, etc.) 45serving as an element other than the silicon microphone 32 are mountedon the mounting board 42 made of plastic or ceramic.

A ground pattern 46 and a microphone signal output pattern 47 are formedon a back side of the mounting substrate 42. In a mounted state, thesilicon microphone 43 is mounted on the mounting board 42 as shown inFIG. 4. The vibrating film (vibrating electrode) 33 serving as one ofelectrodes of the condenser is electrically connected to the electroniccomponent 45 via a bonding wire 44 a. Further, the electronic component45 is electrically connected to a wiring pattern 60 b on the mountingboard 42 via a bonding wire 44 c. The fixed electrode 31 serving as theother of the electrodes of the condenser is electrically connected tothe wiring pattern 60 a on the mounting board 42. Further, wiringpatterns 60 a and 60 b are electrically connected to the ground pattern46 and the microphone signal output pattern 47 formed on the back sideof the mounting board 42 via lines L1 and L2 provided in the mountingboard 42, respectively.

After the electretization process is completed, a shield case 41 isattached to the mounting board 42. A wide opening 49 is provided in theshield case 41 as an acoustic hole for guiding a audio wave.

As described above, according to the present embodiment, in thecondenser microphone made by micromachining a silicon substrate, thedielectric film 32 is electretized with high accuracy to a targetretained charge amount. Subsequently, the ground is disconnected, andthe static elimination process is performed. As a result, unnecessaryelectric charges can be eliminated without the change in the retainedcharge amount. Thus, the change in microphone sensitivity and the changein the retained charge amount of the charged dielectric film can bereduced.

In the present embodiment, the method for electretizing the MEMSmicrophone chip is explained. However, the embodiment may be applied toan electretization on a wafer scale or to an electretization aftermounting.

The present invention is not limited to the microphone chip described inthe embodiment, and may be modified or applied by those skilled in theart based on the descriptions of the specification or well-knowntechniques, and such modifications and applications shall fall a scopeof the present invention.

The present patent application is based on Japanese Patent Application(Japanese Patent Application No. 2008-147023) filed on Jun. 4, 2008, theentire contents of which are incorporated herein by reference.

A static eliminator of the embodiment can reduce a change in a retainedcharge amount of a electretized dielectric film and also eliminateunnecessary electric charges from the electretized device. The staticeliminator can be applied to static elimination of various devices suchas an MEMS microphone.

DESCRIPTIONS OF THE REFERENCE NUMERALS AND SYMBOLS

-   -   1 IONIZER    -   3 INSULATING SUBSTANCE    -   4 CONDUCTIVE SUBSTANCE    -   5 GROUND    -   11 NEEDLE ELECTRODE    -   12 HIGH VOLTAGE POWER SOURCE    -   31 FIXED ELECTRODE    -   32 DIELECTRIC FILM    -   33 VIBRATING FILM    -   34 SILICON SUBSTRATE    -   35 ACOUSTIC HOLE    -   36 AIR GAP    -   37 SPACER    -   41 SHIELD CASE    -   42 MOUNTING BOARD    -   43 SILICON MICROPHONE (SEMICONDUCTOR DEVICE)    -   45 ELECTRONIC COMPONENT    -   46 GROUND PATTERN    -   47 MICROPHONE SIGNAL OUTPUT PATTERN    -   49 OPENING    -   60 a, 60 b WIRING PATTERN

1. A static eliminator comprising: an ionizer comprising a power sourceand an electrode which applies ions to an electretized dielectric filmto eliminate electric charges formed on the electretized dielectricfilm, wherein the electretized dielectric film is in an electricallynon-grounded state when the electric charges on the electretizeddielectric film are eliminated.
 2. The static eliminator according toclaim 1, comprising: a conductive substance, wherein the conductivesubstance induces an electric field on the dielectric film and is in anelectrically non-grounded state when the electric charges on theelectretized dielectric film are eliminated.
 3. The static eliminatoraccording to claim 2, wherein the conductive substance is connected tothe dielectric film.
 4. The static eliminator according to claim 1,wherein the electrode is a needle electrode.
 5. The static eliminatoraccording to claim 1, wherein the dielectric film is formed on a secondfilm, wherein the second film is formed on a substrate, and wherein athird film is formed opposed to the second film.
 6. The staticeliminator according to claim 5, wherein the second film is a vibratingfilm, and wherein the third film is a fixed film.
 7. The staticeliminator according to claim 5, wherein the substrate is in anelectrically non-grounded state when the electric charges on theelectretized dielectric film are eliminated.
 8. A microphoneelectretizing apparatus for electretizing a dielectric film provided ina microphone, the apparatus comprising: an ionizer which electretizesthe dielectric film and eliminates electric charges on the electretizeddielectric film, wherein, the electretized dielectric film, and asupport for supporting the dielectric film or a vicinity thereof are inan electrically non-grounded state when the electric charges on theelectretized dielectric film are eliminated.
 9. The microphoneelectretizing apparatus according to claim 8, wherein the dielectricfilm is formed on a second film, the second film is formed on asubstrate, a third film is formed opposed to the second film.
 10. Themicrophone electretizing apparatus according to claim 9, wherein thesecond film is a vibrating film, and wherein the third film is a fixedfilm.
 11. The microphone electretizing apparatus according to claim 9,wherein the substrate is in an electrically non-grounded state when theelectric charges on the electretized dielectric film are eliminated. 12.A static eliminating method comprising: eliminating electric charges onan electretized dielectric film by applying ions to the electretizeddielectric film using the ionizer while the dielectric film is in anelectrically non-grounded state.
 13. A microphone electretizing methodcomprising: electretizing a dielectric film by applying ions to thedielectric film using an ionizer; and eliminating electric charges onthe electretized dielectric film by applying ions to the electretizeddielectric film using the ionizer while the dielectric film is in anelectrically non-grounded state.
 14. The microphone electretizing methodaccording to claim 13, wherein the dielectric film is formed above awafer, and wherein said electretizing is performed on the wafer.